1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride polymer while controlling the formation of polymer scales.
2. Description of the Prior Art
In processes for subjecting vinyl chloride monomer or a mixture of vinyl chloride monomer and the other vinyl monomer to suspension polymerization or emulsion polymerization in the presence of a polymerization catalyst in an aqueous medium, there is the problem that polymers in the form of scales are deposited on the inner wall surface of a polymerization vessel and the areas with which monomers contact during polymerization, such as the surface of a stirrer among attached equipments for the polymerization vessel, that is, on the inner surfaces of the polymerization vessel. When the polymer scales are deposited on the inner surfaces of the polymerization vessel, as mentioned above, the efficiency of heat transfer in the wall of the polymerization vessel and the yield of polymer are lowered and in addition thereto, the deposited scales are peeled off from the polymerization vessel and mixed into the product to deteriorate the quality thereof, for example, by causing so-called fisheyes. Further, in order to remove the scales, it requires great deals of labor and time, resulting in many disadvantages, such as lowering in the operation rate of the polymerization vessel.
Especially in the case of suspension polymerization, the scales deposited inside the polymerization vessel is low in porosity (porousness) as compared to a normal polymer simultaneously produced, so that when the scales are peeled off and mixed into the polymer, the affinity of the normal polymer with plasticizers and other forming additives which are added in forming and processing steps is lowered to generate particles (so-called fisheyes) partially incompatible with the final product. Furthermore, when polymerization is repeated intact without removing the scales from the polymerization vessel, even the shape inside the polymerization vessel is changed due to the built-up of scales and as a result, especially when the built-up of scales on the stirrer becomes large, the whole suspension system is remarkably affected. Thus, the shape, particle size distribution, yield, etc. of the resulting polymer may be affected.
From the above reasons, it is indispensable to periodically remove the scales from the inner surfaces of the polymerization vessel. For this purpose, it requires to stop or suspend the operation of the polymerization vessel, resulting in increasing the cost directly irrespective of the production of the polymer. Removal of the scales is generally effected by mechanically peeling off the scales from the inner surfaces, striking a high speed jet stream of water to the same or dissolving the same in an organic solvent. In part of such a lot of work, the workers should enter in the vessel to carry out the work. However, since this accompanies the afraid that the workers touch vinyl chloride monomer which is doubted on cancer-causing, it is not desirable at all. Further, the mechanical removal of scales from the polymerization vessel causes a high possibility to damage the inner surfaces and the damage accelerates the formation of scales in the subsequent polymerization. Besides, the method of removing the scales using the solvent requires an excess cost for a solvent recovery equipment, and the operation and maintenance thereof.
Accordingly, as methods for preventing such a deposition of polymer scales, there is known a method forming a coating film of a scale deposition preventive agent and a variety of scale deposition preventive agents are proposed. These scale deposition preventive agents proposed include, for example, dyes or pigments [Japanese Patent Publication (kokoku) No. 45-30835]; polar organic compounds [Japanese Patent Publication (kokoku) No. 45-30343]; poly-aromatic amine compounds [Japanese Pre-examination Patent Publication (kokai) No. 53-23381]; condensates of a phenol compound with formaldehyde or benzaldehyde [Japanese Pre-examination Patent Publication (kokai) No. 54-36389].
The formation of coating films of these scale deposition preventive agents are effected in the following manner. After the end of polymerization, a polymer slurry produced is withdrawn from a polymerization vessel and then the inside of the polymerization vessel is washed with water. Thereafter, the following three steps are effected: (1) a coating solution of the scale deposition preventive agent is spray coated on the areas with which vinyl chloride monomer contacts during polymerization, such as the inner wall surface of the polymerization vessel, from spray nozzles provided on the upper part of the gas phase zone inside the polymerization vessel (coating step), (2) the coated surface after spray coating is dried to obtain a dry coating film (drying step), and (3) the dry coating film is washed with water (water washing step). In the spray coating, the spray has been generally formed by supplying the coating solution under pressure to the spray nozzles.
After forming the coating film of the scale deposition preventive agent in the above manner, vinyl chloride monomer and the other raw materials are charged in the polymerization vessel and then polymerization is effected to produce a vinyl chloride polymer. In recent years, in order to improve productivity, it is desired to shorten the time taken to recycle a series of processes comprising the step of forming the polymer scale deposition preventive coating film, the step of charging raw materials, the step of polymerization reaction, the step of recovering unreacted monomer and withdrawing the polymer slurry, and the step of washing the inside of the polymerization vessel with water. As a part thereof, shortening of the time taken to form the coating film is required.
At the same time, for the purpose of improving productivity, there is recently employed a polymerization process using a large-sized polymerization vessel having a capacity of 40 m.sup.3 or more and further being capable of shortening polymerization time. In such a polymerization process capable of shortening polymerization reaction time, a procedure increasing the amount of a polymerization initiator is employed. Since an increase in the amount of the polymerization initiator results in a large quantity of heat generated per unit time, it requires an improvement in efficiency of heat removal. Typical means for heat removal include a cooling jacket and a reflux condenser. On large-sizing the capacity of a polymerization vessel to 40 m.sup.3 or more, there occurs a necessity for increasing a rate of heat removal by the reflux condenser since heat removal by the jacket is in sufficient. However, with an increase in the load of heat removal on the reflux condenser, bubbling of slurry inside the polymerization vessel is enhanced and the slurry overflows into the reflux condenser. As a result, the heat removal capacity of the reflux condenser is reduced or deposition of polymer scales takes place. In some cases, pipelines are clogged by the overflowed polymer particles and the operation of the condenser becomes impossible. Further, on using the condenser from the beginning of polymerization, there occurs such a problem that the particle size of the resulting polymer particles becomes coarse or a bubble-like polymer is produced.
Accordingly, in case where the polymerization process shortening polymerization reaction time by using a large-sized polymerization vessel, the capacity of heat removal can not help being insufficient when only the jacket and the reflux condenser are used and the insufficient capacity of heat removal has to supplement with the other cooling means. As such other cooling means, an internal cooling device, such as a cooling coil, a draft tube or a cylindrical cooling baffle is provided in the polymerization vessel.
Now, when a coating film of a polymer scale deposition preventive agent is formed, the spray coating having a high efficiency as mentioned above is conventionally used. In the conventional spray coating, however, the coating solution can not be sufficiently coated on the areas to be shadowed when viewed from the spray nozzles inside the polymerization vessel, e.g., the surfaces of baffles provided in the polymerization vessel, facing the inner wall of the polymerization vessel, and as a result, the coating is liable to become uneven. Especially when the inside of the polymerization vessel has a complicated structure due to the provision of internal cooling devices, such insufficiently coated areas are increased. Thus, with an increase inthe number of polymerization batches to be repeated, polymer scales are more deposited on the areas where no sufficient coating films are formed. As a countermeasure therefor, the coating solution is sufficiently sprayed so as to cause no uneven coating, i.e., the coating solution of a polymer scale preventive agent is coated in a large quantity over a long time in the coating step. As a result, however, an excess quantity of spray coating is effected on the other inner surfaces and the drying time taken for the drying step must be extended with an increase in the quantity of the coating solution, and also, since the coating solution of a polymer scale deposition preventive agent remains in a large quantity in the washing step, the washing time must be extended in order to remove the remaining coating solution.
Such a long extension of the time taken to form the coating films results in the disadvantage that it goes against the above requirement for shortening the polymerization time.
In addition, the use of the polymer scale deposition preventive agent coating solution in a large quantity results in adverse affects, such as a remarkable initial-discoloration in the vinyl chloride polymer products obtained by polymerization and an increase in colored foreign matter ascribable to the scale preventive agent present in the products.
Furthermore, polymer scales are deposited on even the areas causing no uneven coating, with an increase in the number of polymerization batches. Especially the areas near the interface between the gas phase zone and the liquid phase zone during polymerization are the areas where scales are readily deposited by, for example, a decrease in flowability due to a factor such as bubbling. Thus, a further improvement in coating at said areas is required.